NECTAR CONSUMERS - Nectaries and Nectar - page 312

factor influencing the motivational state of nectar-feeding ants is colony
starvation, when the ants are more inclined to accept dilute food (10% su-
crose), and the food intake rate and the extent of crop filling both increase
(Josens & Roces, 2000). Viscosity may be less of a problem for more primi-
tive ponerine ants, because they lack a crop and transport nectar as a drop
between the mandibles (Paul & Roces, 2003). The ponerine ant Pachycondyla
villosa licks sugar solutions and its maximum energy intake rate occurs at a
higher concentration than that of the formicine C. rufipes , which sucks fluid
food (Table 3).

4 VERTEBRATE NECTAR CONSUMERS

Endothermic vertebrate pollinators have the advantage of being dependable
over a wide range of seasons and altitudes (Wolf & Gill, 1986). Nectarivorous
birds and bats, in particular, are highly mobile, and their daily and seasonal
tracking of nectar resources adds to their value as pollinators (Fleming,
1992). However, rewarding them represents a significant investment by the
plant, in terms of nectar production and also the substantial floral structures
required to produce and contain it (Fig. 4). Extrafloral nectaries are generally
small, and it is unlikely that the energetic returns from foraging on them will
be worthwhile for vertebrates. Yet a remarkable exception is seen in Austra-
lian honeyeaters, which are attracted to the large red extrafloral nectaries of
Acacia terminalis , contacting the nectarless flowers and transferring pollen
while they forage (Knox et al., 1985; Stone et al., 2003).

Vertebrate nectarivores are best represented in the tropics and the southern
hemisphere, where major radiations of plants have not been accompanied by
corresponding bee diversity: this contrasts with the domination of the northern
hemisphere pollination systems by generalist social bees (Johnson & Steiner,
2000). Both diurnal birds and nocturnal marsupials commonly visit the un-
specialized brush flowers of Eucalyptus (Myrtaceae) and Banksia (Protea-
ceae) in Australia, and rodents in South Africa also visit Proteaceae ( Protea
species). Sussman and Raven (1978) considered pollination by non-flying
mammals to be an archaic system, which persists only in the absence of bat
pollinators, and this may be true of some regions (see Carthew & Goldingay,
1997). An interesting example is seen in the large-flowered Strelitziaceae,
where the basal genus Ravenala is pollinated by lemurs in Madagascar, and
the more derived Phenakospermum and Strelitzia are pollinated by bats in
South America and birds in South Africa, respectively (Kress et al., 1994).